1. Technical Field
The present invention relates generally to the chemical mechanical polishing of printed circuit boards, and more particularly, to a planar head dampening system for a chemical mechanical polishing system.
2. Related Art
The strict process requirements of a chemical mechanical polishing system, used in the manufacture of printed circuit boards, necessitate the use of a compressible polishing device and the application of a low downward pressure, ranging from approximately 0.25 to 10 psi. These requirements are essential to the polishing process due to the delicate nature of the surface contours of the workpiece. Use of a less compressible polishing device or attempting to increase the down pressure could eliminate these desired surface contours or otherwise damage the workpiece. Unfortunately, gravity alone is insufficient to hold the polishing device in contact with the workpiece while polishing. Rather, in the absence of an applied down pressure, the polishing device floats above the workpiece without polishing its surface. Therefore, a polishing system utilizing a planar head polishing device having a low durometer compressible core in conjunction with the application of a low down pressure are currently used in the industry.
The requisite down pressure is typically supplied by a compressed air source. However, when the surface of the planar head polishing device was upgraded to a more aggressive material, a destructive phenomenon known as "high frequency cycling" was encountered. High frequency cycling results when the planar head is brought into contact with the workpiece, and due to its coarse polishing surface, the planar head begins to grip the surface of the workpiece and "bite-in." This causes the compressible inner core of the planar head to deform, forcing it in the opposite direction intended. Due to the compressible nature of the air providing the down pressure, the upward force exerted by the deformed planar head produces a rebounding effect. These fluctuations in contact between the polishing device and the workpiece produce proportionate fluctuations in the planar head impression and the process rate of the workpiece. Additionally, premature failure of the planar head lowering device, inconsistent surface conditions of the planar head and non-uniform planarization of the workpiece surface can result.
The high frequency cycling appears to be attributed to the combination of the low durometer inner core of the planar head and the low down pressure. In an attempt to remedy the problem, planar heads composed of different durometer materials were tested. However, these alternative materials created further processing problems and, therefore, offered no solution. In the alternative, various shock absorbers were considered to eliminate the effects of the compressed air. However, none produced a regressive non-compressible system, which would allow the pressure to remain constant throughout polishing. These alternatives were also ineffective since they would be destroyed by the harsh chemical environment of the system.
Therefore, there is clearly a need to develop a chemical mechanical polishing device which eliminates the occurrence of high frequency cycling.